1. Field of the Invention
The present invention relates to an image display apparatus having pixels continuously arranged in the line and column directions. More particularly, the invention relates to an image display method and an image display apparatus for generating display data out of original image data for gradation display, wherein the bits constituting the display data are fewer than those making up the original image data.
2. Description of the Related Art
The liquid crystal display (LCD) apparatus for use with computers is a typical image display apparatus on which images are displayed in gradations on the basis of the original image data made up of digital signals. This type of image display apparatus works primarily by generating display data from the original image data, with the display data comprising fewer bits (X bits, e.g., 3 bits) than the original image data having Y bits (e.g., 6 bits). The display data thus generated is used as the basis for driving each of the pixels of a display panel for gradation display. Illustratively, whereas six-bit original image data permits image display at 64 gray levels, each pixel is capable of a display only at eight gray levels if the pixel is driven on the basis of three-bit display data. The display image gray level count is enhanced by a known image signal processing method whereby the high-order X bits of the original image data are used as display data and the low-order Y−X bits (e.g., 3 bits) as error data. The gradations in which to display images by the low-order Y−X bits are diffused among the display data for each of the pixels so that the gradations corresponding to the low-order Y−X bits will be simulated by a plurality of pixel groups.
FIGS. 11A through 11C and FIGS. 12A through 12E show how such a conventional signal processing method diffuses the error between original image data and display data among pixels for simulated gradation display. FIG. 10 indicates a screen of one frame (i.e., 1 screen). Dots in the screen represent a pixel each. In the description that follows, a crosswise arrangement of pixels on the screen will be called a line, and a longitudinal arrangement of pixels will be called a column. In FIG. 10, the pixels on a line K, columns L−1 and L, are indicated as a point each. On the ordinary LCD apparatus, lines are selected sequentially while the columns are fed simultaneously with driving voltages representing display data. The pixels then give display in gradations depending on the supplied driving voltages.
FIGS. 11A through 11C depict a conventional example in which image signals are processed through error diffusion. FIG. 11A shows original image data to be fed to the pixel on the line K, column L, the pixel being shown as a point in FIG. 10. The original image data of this example is made up of six bits, with D0 representing the least significant bit and D5 denoting the most significant bit in FIG. 11A. The conventional signal processing method cited here utilizes as error data the low-order three bits (D2′, D1′, D0′) derived from the processing of the six-bit original image data given to the pixel on the column L−1 preceding the column L (see FIG. 11B). The three-bit error data is added to the original image data of six bits in FIG. 11A. Of the six-bit data resulting from the addition shown in FIG. 11C, the high-order three bits (D5′, D4′, D3′) are used as display data. The display data is used as the basis for supplying driving voltages to the pixel on the line K, column L, so that the pixel is driven to predetermined gray levels. The low-order three bits (D2″, D1″, D0″) are used as the error data for the pixel on the next column L+1.
FIG. 13A shows eight pixels arranged in a line. Suppose that the eight pixels in FIG. 13A are all fed with the same original image data and that the low-order three bits (D2, D1, D0) are “001.” In that case, the error diffusion processing method of FIGS. 11A through 11C works as follows. The three-bit error data is added consecutively to the low-order three bits of the original image data. At the pixel in the position 23 (i.e., eighth pixel), the most significant bit (D2″) of the error data generates a carry, adding “1” to the least significant bit (D3′) of the display data. As a result, the eighth pixel in FIG. 13A gives a display one gray level higher than that of the remaining seven pixels. This allows a set of eight pixels to simulate the gray level that is to be represented by the low-order three bits of the original image data.
One disadvantage of the error diffusion processing in FIGS. 11A through 11C is a degradation of image quality that can happen when an image of the same gray level is continuously displayed as a still picture in a fixed area of the screen, i.e., when the original image data corresponding to the pixels constituting a fixed area remains the same for an extended period of time. For example, suppose that as shown in FIG. 13B, the same original image data with its low-order three bits of “001” continues to be fed to a pixel group comprising 8×4 contiguous pixels occupying a certain area. In such a case, the same column continuously has pixels in which “1” is added to the least significant bit of the three-bit display data (indicated as black fill-in in FIGS. 13A and 13B). In the line direction, the black fill-in appears at intervals of eight pixels. The result is a set of fine longitudinal stripes appearing in the still picture of a given gray level.
One solution to the deficiency above is a technique of inter-frame compensation for the error from expressing gray levels with low-order bits of original image data. One such technique is an inter-frame error diffusion processing method disclosed illustratively in Japanese Patent Laid-Open No. Hei 6-118920 (1994).
FIGS. 12A through 12E show how the disclosed method above works for inter-frame diffusion of gradation display error. FIG. 12A indicates original image data corresponding to the pixel on the line K, column L in FIG. 10. As in the example of FIGS. 11A through 11C, the original image data is made up of six bits. This processing method utilizes as inter-frame error data the low-order two bits (D1′, D0′) following the processing of the original image data corresponding to the pixel on the line K, column L−1 preceding the column L (FIG. 12B). The two-bit inter-frame error data is added to the low-order two bits of the original image data shown in FIG. 12A. FIG. 12C illustrates six-bit data derived from the addition. To the data in the fourth most significant bit (D2′) in FIG. 12C is added the inter-frame error data (D2″) corresponding to the pixel in the same position as that of the preceding frame. As shown in FIG. 12E, the high-order three bits resulting from the addition are used as display data for driving the pixel. The low-order two bits (D1″, D0″) are used as the intra-frame error data for the pixel on the next column L+1, while the fourth most significant bit (D2″) is used as the inter-frame error data for the pixel on the line K, column L of the next frame.
In short, the above processing method regards the low-order two bits of the six-bit original image data as inter-frame error data, uses the fourth most significant bit as intra-frame error data, and employs the high-order three bits as display data.
With the conventional processing method for inter-frame error diffusion of FIGS. 12A to 12E, suppose that original image data of the same gray level is applied to all pixels constituting a certain screen area for a predetermined period of time. FIGS. 14A through 14H depict groups of 8×4 pixels giving gradation display under the above condition. FIG. 14A shows a case where the pixels in the pixel group occupying the fixed screen area are fed with the same original image data having the low-order three bits of “000”; FIG. 14B is a case where the pixels are supplied with the same original image data having the low-order three bits of “001”; FIGS. 14C through 14H each indicate a case where the pixels are likewise supplied with the same original image data with its low-order three bits fed consecutively with “1.” It follows that in FIG. 14H, the low-order three bits of the same original image data given to the pixels involved are “111.” In each of FIGS. 14A through 14H, the upper view is a screen corresponding to an odd-numbered frame, and the lower view is a screen corresponding to an even-numbered frame continued temporally to the odd-numbered frame.
In the screens resulting from the signal processing of FIGS. 12A through 12H, each pixel in which “1” is added to the least significant bit of the three-bit display data is painted black, and the pixels not fed with “1” are each indicated as “0.”
The inter-frame error diffusion method above is now compared with the intra-frame error diffusion processing method of FIGS. 11A through 11C. According to the processing method of FIGS. 11A through 11C, if the original image data corresponding to the pixels occupying a certain screen area has the low-order three bits of “001,” the added error data is reflected at intervals of eight pixels as shown in FIG. 13B. According to the processing method of FIGS. 12A through 12E, on the other hand, if the original image data corresponding to the pixels occupying the fixed screen area also has the low-order three bits of “001,” the added error data is reflected at intervals of four pixels as shown in the lower view of FIG. 14B. The latter method provides better gradation quality for the still picture than the case of FIGS. 13A and 13B because the inter-frame error diffusion shortens the distance between the pixels in which the added error is reflected.
However, in FIGS. 14B, 14C and 14D, the pixels to which “1” is added (i.e., black-painted pixels) are identically arranged between adjacent lines and these pixels continue in the column direction within the frame. As a result, longitudinal strips are unavoidable in the still picture. Furthermore, as shown in FIGS. 14D, 14B and 14F, there occurs a drastic difference in the number of “1”-fed pixels between an odd- and an even-numbered frame. This leads to a flicker phenomenon contingent on the gray level of the still picture. In FIG. 14E, for example, the flickers are particularly pronounced depending on the gray level because there are no black-filled pixels in the odd-numbered frame while all pixels are black in the even-numbered frame.
The processing method of FIGS. 12A through 12E requires retaining for a one-frame interval the inter-frame error data about the pixels in the same positions as those of the preceding frame. This entails extremely complicated control procedures regarding data write and read operations.